Ion implantation is today a well established process used in the fabrication of solid state semiconductor devices. For example, see VLSI Fabrication Principles, S. K. Ghandhi, John Wiley and Sons, Inc. (New York 1983). Further elaboration on ion implantation processes as well as on the apparatus used therefor may be found in Ion Implantation Techniques, H. Ryssel and H. Glawischnig, Editors, Springer-Verlag (New York 1982). Basically, ion implantation apparatus comprises a source of ions, means for analyzing the source ions, means for accelerating the ions toward the target substrate and means for scanning the ion beam across the substrate. In the ion source, the dopant species to be implanted is provided in solid, liquid or gaseous form and is ionized. Similarly charged ions are elecgtrostatically extracted and are directed to the ion analyzer wherein they are separated by magnets so as to yield a beam of a particular ionic species. The ion beam which exits the ion analyzer is then again accelerated electrostatically and is directed between the deflection plates of a scanning system towards the target substrate.
The target substrate is typically connected to ground via a current measuring device known as a current integrator, which measures the parameter termed beam current. In order to operate in a stable fashion implantation system require that the ion source produce a certain minimum output. This minimum output corresponds to a typical beam current minimum value on the order of 10.sup.-6 amps. Other parameters being equal, the dosage of the ion implant is directly proportional to the beam current at the target. An effective low dosage implant requires a correspondingly low beam current. However, certain implantation steps in certain semiconductor fabrication processes, such as the conventional threshold voltage adjustment implant in metal oxide semiconductor field effect transistor (MOSFET) manufacture, require beam currents as low as 2.5.times.10.sup.-8 amps so as to yield a dosages of approximately 1 to 5.times.10.sup.10 ions/cm.sup.2. Thus, since the ionization source yields a beam current of at least 10.sup.-6 amps, some beam attenuation means is required in order to achieve 2.5.times.10.sup.-8 amps at the target.
The beam attenuation means conventionally takes the form of a shutter that includes a "variable slit" aperture in the beam path. However, such variable slit shutters create significant processing problems. The impinging ion beam causes sputtering and electrostatic charging of the shutter material, which in turn causes implantation non-uniformities due to beam distortion and contamination. Furthermore, a significantly attenuated beam presents a very narrow section, rendering the x-y scan of the beam on the target a critical, albeit difficult to uniformly control operation. In an effort to overcome the problems associated with performing low dosage ion implants, i.e. dosages less than approximately 1-5.times.10.sup.10 ions/cm.sup.2, the present invention was conceived.